The autonomic nervous system (ANS) is the portion of the nervous system that controls the body's visceral functions, including action of the heart, movement of the gastrointestinal tract, and secretion by different glands, among many other vital activities, in order to maintain homeostasis of the body. Heart rate variability (HRV) measurement, also called heart period variability measurement, is a non-invasive tool for measuring the status of autonomic nervous system. Heart rate variability refers to the measurement of the regulation of the sinoatrial node, the natural pacemaker of the heart by the sympathetic and parasympathetic branches of the autonomic nervous system. The assumption when assessing HRV is that the beat-to-beat fluctuations in the rhythm of the heart provide an indirect measure of heart health, as a dynamic window into autonomic function and balance in sympathetic and vagus nerve activity. Simply put, HRV is the change in the length of time of consecutive heartbeats. As a matter of measurement convenience, a heartbeat is usually measured as the time (in msecs) from the peak of one R wave to the peak of the next. This time is referred to as the RR interval, the interbeat interval (IBI), or the heart period.
The changes in the length of normal RR intervals—that is, their variability—can be determined in different ways. Analysis is most commonly in either the time domain or the frequency domain. Time domain measurements use typical linear means of contrasting the individual RR intervals to the mean interval. An example of the many commonly applied time domain measurements is the standard deviation of the RR intervals. Frequency domain analysis uses non-linear methods such as Fast Fourier Transformation (FFT) or autoregressive analysis to determine at what frequencies the variability lies. This type of analysis has proven valuable because the physiological cause of the variability may be linked to certain frequency bands.
Recently chaos theory has also been applied to HRV analysis. Those arising from a normal sinus rhythm are considered. Ectopic beats and escape beats are not considered normal and are usually dealt with by systematically eliminating and smoothing of the remaining interval. HRV is characterized by four main components: the high frequency (HF) component (0.15 Hz to 0.40 Hz) measures the influence of the vagus nerve in modulating the sinoatrial node. The low frequency (LF) component (0.04 Hz to 0.15 Hz) provides an index of sympathetic effects on the heart, particularly when measured in normalized units. The very low frequency (VLF) component (0.003 Hz to 0.04 HZ) reflects the influence of several factors on the heart, including, for example, chemoreceptors, thermoreceptors, the renin-angiothensin system, and other non-regular factors. Almost all of the variability from a short-term spectral analysis of HRV is captured in these three components. An ultra low frequency (ULF) component (5.003 Hz) can also be observed in the HRV spectrum of a long sample.
The autonomic nervous system is linked and receives information from centers located in the spinal cord, brain stem, hypothalamus, and cerebral cortex. Furthermore, parts of the body send impulses by visceral reflexes into the centers in a dynamic, ongoing, multi-way dialogue, with each organ continuously influencing the other's function. This communication network is based along two major ways: neurological (through the transmission of nerve impulses) and biochemical (via hormones and neurotransmitters).
The two major subdivisions of the transmission system of the ANS (i.e., the sympathetic and parasympathetic) regulate the body in response to an ever-changing internal and external environment. The sympathetic system is known as the “body accelerator.” It activates the body and mind for exercise and work and it prepares the body to meet real or imagined threats to its survival. The parasympathetic system can be compared to a “brake.” When the parasympathetic system is activated, we generally tend to relax and slow down. But each system can have inhibitory effects in some organs and excitatory effects in others. For example, the generally exciting sympathetic system inhibits the digestive musculature and by exciting the microvascular arteriolar sphincters, reduces the digestive blood flow. Conversely, the enervating parasympathetic system is extraordinarily exciting for the digestive system, and increases the visceral blood circulation.
Pacing of the stomach and other portions of the gastrointestinal (GI) tract via electrical pulses has been experimented with for some time. Most of the experimentation has been oriented toward improving the gastric emptying usually by attempting to speed up the transit time of food moving through the GI tract (for failure to thrive, gastroparesis, or pseudo-obstruction) or of relieving the neurally mediated symptoms associated with gastroparesis.
U.S. Pat. No. 5,423,872 (Jun. 3, 1995) to Cigaina for “Process and Device for Treating Obesity and Syndromes Related to Motor Disorders of the Stomach of a Patient” describes an implantable gastric electrical stimulator at the antrum area of the stomach which generates sequential electrical pulses to stimulate the entire stomach, thereby artificially modifying the natural gastric motility and emptying or slowing down food transit through the stomach. U.S. Pat. No. 5,423,872, however, has the inherent disadvantage that it is a stimulation device solely, and does not incorporate sensed triggered stimulation other than that of manual cycling provided by magnetic application, which potentially wastes energy by applying stimulation when it is not therapeutically required.
U.S. Pat. No. 5,690,691 (Nov. 25, 1997) to Chen et al. for “Gastro-intestinal Pacemaker Having Phased Multi-Point Stimulation” describes a portable or implantable gastric pacemaker employing a number of electrodes along the greater curvature of the stomach for delivering phased electrical stimulation at different locations to accelerate or attenuate peristaltic movement in the GI tract. The Chen et al. patent additionally provides a sensor electrode or a stimulation electrode wherein the response of an organ to an electrical stimulation pulse is sensed for delivering stimulation to a plurality of electrodes to provide phased electrical stimulation. However, Chen et al. is specifically directed to phase stimulation which progresses through the plurality of electrodes located along the peristaltic flow path and specifically senses the response of the organ to the electrical stimulation, but does not provide sensing of heart rate.
U.S. Pat. No. 5,836,994 (Nov. 17, 1998) to Bourgeois for “Method and Apparatus for Electrical Stimulation of the Gastrointestinal Tract” describes an implantable gastric stimulator which incorporates direct sensing of the intrinsic gastric electrical activity by one or more sensors of predetermined frequency bandwidth for application or cessation of stimulation based on the amount of sensed activity. The Bourgeois sensor does not contain algorithms to determine gastric rate variability.
U.S. Pat. No. 5,861,014 (Jan. 19, 1999) to Familoni for “Method and Apparatus for Sensing a Stimulating Gastrointestinal Tract On-Demand” relates to an implantable gastric stimulator for sensing abnormal electrical activity of the gastrointestinal tract so as to provide electrical stimulation for a preset time period or for the duration of the abnormal electrical activity to treat gastric rhythm abnormalities. Familoni also addresses recording of abnormal activity for a preset time period, but does not address altering of a normal gastric activity to achieve a variable result such as treatment for obesity.
U.S. Pat. No. 6,600,953 to Flesler et al. for “Acute and Chronic Electrical Signal Therapy for Obesity,” addresses applying a non-excitatory electrical field to the stomach such that it increases the level of contraction of muscle tissue of the body of the stomach and decreases a cross sectional area of a portion of the stomach for a substantially continuous period of greater than about 3 seconds. Flesler applies a non-excitatory electrical field denoted as an Excitable-tissue control (ETC) signal that modifies the response of one or more cells to electrical activation so as to delay or prevent emptying of the stomach by increasing the level of contraction of stomach muscle(s) and thus narrowing the cross-sectional area of the stomach. Flesler invokes various singular sensors to initiate or terminate the ETC signal and incorporates patient activation and programmable timed periods of activation to attempt to achieve an acceptable life for the implantable device. The sensors are not intended to measure either HRV or GRV.
U.S. Pat. No. 6,571,127 to Ben-Haim et al. for “Method of Increasing the Motility of a GI Tract,” addresses application of an non-excitatory electrical field excitable-tissue control signal to increase the contractile force and/or motility of a GI tract. The non-excitatory electric field is applied to reinforce (strengthen) a forward propagating wave and/or to inhibit (reduce) the response to the activation signal of a returning wave. Again, Ben-Haim does not provide sensors to measure HRV or GRV.
U.S. Pat. No. 5,928,272 shows a device and method to treat epilepsy, where electrical stimulation therapy is triggered based upon heart rate. This patent shows a correlation between a sudden change in heart rate and the potential for an epileptic seizure. Noting this sudden change, therapy can be induced to a nerve for the acute situation of halting or minimizing a seizure. The present application, on the other hand, has been recognized to be a treatment for different disease states outside the area of epilepsy, and is based upon heart rate variability analysis that does not involve a quick sudden rate of change as would be seen just prior to the onset of a seizure.
More recently, U.S. patent application Ser. No. 10/627,908 (filed Jul. 25, 2003) provides methods whereby an electrical stimulation device is implanted on the small intestines or lower bowel. All of these patents and patent applications, as well as all patents, patent applications, and publication cited herein, are hereby incorporated by reference in their entireties.
Similarly, in U.S. Patent Publications 2003/0181958 and 2003/0181959, Dobak describes a nerve stimulator which stimulates nerves of the sympathetic nervous system of body. In comparison, the present invention is aimed toward regulating the parasympathetic system of the body triggered from HRV.